1. Field of the Invention
This invention relates to the removal of epitaxial layers and, more particularly, to the removal of epitaxial layers from a gadolinium garnet substrate.
2. Art Background
Magnetic bubble devices are frequently used as memories in the computer industry. Of the numerous steps required to produce a finished device, one of the most critical is the growth of an epitaxial layer of magnetic garnet material on an appropriate substrate.
Typically, a non-magnetic substrate is prepared on which the epitaxial layer is deposited. This epitaxial layer is deposited on the non-magnetic substrate in a well known liquid phase epitaxy process (i.e. the epitaxial layer assumes the same crystallographic structure as the substrate). Liquid phase epitaxy is accomplished, in part, by dissolving oxides of the various metal and rare earth ions that are desired in the epitaxial layer in a growth melt (see for example, U.S. Pat. No. 4,190,683).
In practice, a number of difficulties associated with the growth of an epitaxial layer on the substrate have precluded the reduction of the cost of each device. Specifically, the formation of mesas and pits on the epitaxial layer render wafers or portions thereof unuseable. Mesa defects are irregularities in the epitaxial layer which typically project between 2 and 20 microns above the surface of the epitaxial layer. In addition, pitting of the epitaxial layer may occur when lead oxide used in the melt of epitaxial material accidently drips onto the epitaxial layer as it is being withdrawn from the melt solution. The presence of these defects renders the epitaxial layer unusable, since it has been found that these defects may stop bubble movement and/or generate spurious magnetic bubbles thereby introducing errors.
A variety of processes have been devised to lower the incidence of defect formation. For example, one common method is to spin the substrate after it emerges from the growth melt. Additionally, attempts have been made to reduce mesa formation by growing the epitaxial layer with the substrate tilted at an angle (i.e. at 5 degrees). Moreover, other attempts to preclude defect formation include stirring the growth melt before substrate immersion and during the growth process, and combining other methods, such as for example tilting the substrate, to minimize defects. (See for example Giess et al, IMB technical disclosure bulletin, 16(9), 3049 (1974); and U.S. Pat. No. 4,277,519).
Despite attempts to minimize the formation of irregularities such as mesas and pits a certain percentage of bubble wafer area is defective after epitaxial growth. Presently, wafers on which epitaxial defects are discovered are either discarded or repolished. Inasmuch as the wafer constitutes a substantial portion of the overall cost of each bubble device, discarding the entire wafer is wasteful and expensive. In addition, polishing a wafer to remove the epitaxial layer in order to begin the growth process anew is both time consuming and expensive. Accordingly, there exists a need to provide a cost effective means whereby a previously grown epitaxial layer can be removed from a substrate without the necessity of reprocessing or polishing the wafer before a new layer is grown.